linux-kernel - Re: [PATCH 05/18] arm64: kbuild: reserve reg x18 from general allocation by the compiler

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Message-ID: <CAKwvOd=SZ+f6hiLb3_-jytcKMPDZ77otFzNDvbwpOSsNMnifSg@mail.gmail.com>
Date:   Fri, 18 Oct 2019 10:32:20 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Dave Martin <Dave.Martin@....com>,
        Kees Cook <keescook@...omium.org>,
        Laura Abbott <labbott@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        kernel-hardening@...ts.openwall.com,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 05/18] arm64: kbuild: reserve reg x18 from general
 allocation by the compiler

On Fri, Oct 18, 2019 at 9:11 AM Sami Tolvanen <samitolvanen@...gle.com> wrote:
>
> From: Ard Biesheuvel <ard.biesheuvel@...aro.org>
>
> Before we can start using register x18 for a special purpose (as permitted
> by the AAPCS64 ABI), we need to tell the compiler that it is off limits
> for general allocation. So tag it as 'fixed',

yep, but...

> and remove the mention from
> the LL/SC compiler flag override.

was that cut/dropped from this patch?

>
> Link: https://patchwork.kernel.org/patch/9836881/

^ Looks like it. Maybe it doesn't matter, but if sending a V2, maybe
the commit message to be updated?

> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>

If sending a V2 with the above cleaned up, you may also include:
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

I like how this does not conditionally reserve it based on the CONFIG
for SCS.  Hopefully later patches don't wrap it, but I haven't looked
through all of them yet.

> ---
>  arch/arm64/Makefile | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
> index 2c0238ce0551..1c7b276bc7c5 100644
> --- a/arch/arm64/Makefile
> +++ b/arch/arm64/Makefile
> @@ -55,7 +55,7 @@ endif
>
>  KBUILD_CFLAGS  += -mgeneral-regs-only $(lseinstr) $(brokengasinst)     \
>                    $(compat_vdso) $(cc_has_k_constraint)
> -KBUILD_CFLAGS  += -fno-asynchronous-unwind-tables
> +KBUILD_CFLAGS  += -fno-asynchronous-unwind-tables -ffixed-x18
>  KBUILD_CFLAGS  += $(call cc-disable-warning, psabi)
>  KBUILD_AFLAGS  += $(lseinstr) $(brokengasinst) $(compat_vdso)
>
> --
> 2.23.0.866.gb869b98d4c-goog
>


-- 
Thanks,
~Nick Desaulniers